1. Field of the Invention
This invention relates to an EMI shield for an enclosure for an electronic device. More particularly, the invention relates to an EMI shield designed to permit air flow through the EMI shield to cool the electronic device and designed to attenuate the passage of very high frequency electromagnetic radiation generated by the electronic device inside the enclosure.
2. Description of the Related Art
In computing systems, computer peripheral electronic devices, such as disk drives and tape drives, are mounted in shelf frames containing a plurality of the devices. Each peripheral device is mounted in an enclosure, and the enclosure is then mounted in the shelf frame. A plurality of frames may be mounted in a cabinet. It is very important to provide an EMI shielding enclosure around each of the peripheral devices so that electromagnetic radiation generated by the devices is substantially contained within the enclosure. Ideally, the enclosure for each peripheral device is designed as a Faraday cage.
The EMI shielding enclosure around each device would be most easily accomplished by providing a six-sided metal box with no holes. However, this is not practical, as the peripheral device inside the box must make electrical contact with circuitry outside the box. Further, the peripheral device will generate heat, and therefore each device must be cooled by air flow through the box.
A very successful solution to the problem of providing EMI shielding, while providing electrical connections and air flow for cooling, is shown in U.S. Pat. No. 5,483,423, entitled "EMI Shielding for Components" and issued to Mark S. Lewis et al on Jan. 9, 1996. As shown in the Lewis et al patent, the conductive enclosure for the peripheral device has an open back end that permits the peripheral device to plug into a back plane at the back of a shelf frame or cabinet. The other five walls of the enclosure are conductive walls. When the enclosure slides up against a gasket around the back plane and the back plane is a grounded, a sixth, EMI shielding wall for the enclosure is formed. Air flow through the enclosure is provided by slots, or vents, in the front wall of the enclosure and holes in the back plane. Air flow can then be drawn in through the vents at the front of the enclosure and out through the back plane at the back of the enclosure.
Today, the oscillators and clocks in some electronic devices in these EMI shielding enclosures are operating at 500 megahertz or greater. The harmonics of the electromagnetic radiation from these devices may contain significant power and present a threat of high frequency electromagnetic interference with nearby electronic equipment. One design criteria accepted in the industry is that an enclosure with EMI shielding should shield the outside world from electromagnetic radiation originating at a source inside the enclosure up to frequencies including the 5.sup.th harmonic of the fundamental frequency of the source. By this criteria, the enclosure must shield the outside world from electromagnetic radiation up to 2.5 gigahertz. Another standard in electromagnetic shielding is that the shield should provide for protection from frequencies equal to 5 divided by the rise time T.sub.r times .pi. (5/T.sub.r .pi.). By this criteria, a peripheral device having internal signals of 500 megahertz might have to be shielded with an enclosure capable of shielding for radiation up to frequencies of about nine gigahertz.
To provide for air flow through the enclosure, openings through the walls of the enclosure have been kept small. In the past, an opening less than 1/4 inch in diameter through the enclosure wall was small enough to prevent any significant electromagnetic radiation from getting outside the enclosure. Such an opening is not effective at the high frequencies discussed above. One technique that can be used to provide air flow and, at the same time, provide electromagnetic shielding at high frequency are metal honeycomb screens having a predetermined fine mesh and predetermined thickness. These honeycomb metal screens make use of a tunnel attenuation effect to prevent electromagnetic radiation from escaping an EMI shielding enclosure.
Tunnel attenuation or waveguide attenuation provides very effective attenuation of electromagnetic radiation at very high frequencies. The cutoff frequency for such a waveguide attenuator is the frequency at which the wavelength of the emission is approximately equal to twice the diameter of the waveguide opening. For example, for a wavelength of five gigahertz, which is 2.4-inches, a 1.2-inch diameter hole would just begin to attenuate the five gigahertz signal. Below cutoff, the shielding effectiveness in attenuation is given by the expression:
Shielding Effectiveness=32T.div.D, where T is the length of the waveguide and D is the diameter of opening of the waveguide.
With the above criteria in mind, a metal honeycomb mesh could be selected for the front wall of an enclosure by the diameter of the holes in the honeycomb mesh and the thickness of the honeycomb. The difficulty in solving the EMI shielding and air flow problem in this manner is that the honeycomb metal shield is extremely expensive and is also quite heavy for commercial applications. Therefore, what is needed is a lightweight, low cost electromagnetic shield with good airflow for application to an enclosures for electronic devices.